1. Field of the Invention
The present invention relates to an electromagnetic flowmeter that includes a measurement pipe wherein a magnetic pole core is arranged and a lining member that covers the inner wall face of the measurement pipe, and relates particularly to an electromagnetic flowmeter that can effectively lock the lining member.
2. Description of the Related Art
FIG. 13 is a vertical cross sectional view of the structure of a measurement pipe for an electromagnetic flowmeter having a small diameter in a first related art. FIG. 14 is a transverse, cross sectional view of the measurement pipe in FIG. 13.
In FIGS. 13 and 14, a cylindrical measurement pipe 2 made of stainless steel (for example) has flange portions 1A and 1B at each end respectively, and centrally formed insertion holes 3A and 3B. Magnetic pole cores 4A and 4B having cylindrical shapes, for example, are inserted into the insertion holes 3A and 3B and are securely welded to outer ends 5A and 5B of the insertion holes 3A and 3B.
The magnetic pole cores 4A and 4B are disposed so that gaps 6A and 6B are formed near their distal ends when they are inserted into the insertion holes 3A and 3B in the measurement pipe 2, while distal ends 7A and 7B are maintained at like positions relative to an inner wall face 8 of the measurement pipe 2.
A lining composed, for example, of a fluoroplastic is applied to the inner wall face-8 of the measurement pipe 2, the inner distal faces of the magnetic pole cores 4A and 4B, and the gaps 6A and 6B to provide a lining member 9.
Further, insertion holes 10A and 10B, into which detection electrodes (not shown) are to be inserted, are formed on a line that connects the magnetic pole cores 4A and 4B and in a direction perpendicular to the center line of the measurement pipe 2.
Further, cylindrical electrode attachment portions 11A and 11B for fixing the detection electrodes, are securely welded to the outer wall of the center portion of the measurement pipe 2 perpendicular to the magnetic pole cores 4A and 4B.
FIG. 15 is a vertical, cross sectional view of the structure of the measurement pipe of an electromagnetic flowmeter having a small diameter in a second related art. FIG. 16 is a transverse cross sectional view of the measurement pipe in FIG. 15.
In FIGS. 15 and 16, a measurement pipe 12 made of stainless steel, for example, is a cylindrical spool pipe that has flange portions 11A and 11B at its two ends and centrally formed insertion holes 13A and 13B. Magnetic pole cores 14A and 14B having cylindrical shapes, for example, are inserted into the insertion holes 13A and 13B and are securely welded to outer ends 15A and 15B of the insertion holes 13A and 13B.
When the magnetic pole cores 14A and 14B are inserted into the insertion holes 13A and 13B in the measurement pipe 12, distal ends 17A and 17B are maintained in the same plane as an inner wall face 18 of the measurement pipe 12.
Coil bobbins 113A and 113B, around which coils 114A and 114B are wound, are fitted over the magnetic pole cores 14A and 14B.
A lining made, for example, of a fluoroplastic is applied to the inner wall face 18 of the measurement pipe 12 and the distal ends 17A and 17B of the magnetic pole cores 14A and 14B, so as to provide a lining member 19.
Further, insertion holes 110A and 110B, into which detection electrodes (not shown) are to be inserted, are formed on a line that connects the magnetic pole cores 14A and 14B and in a direction perpendicular to the center line of the measurement pipe 12.
Furthermore, cylindrical electrode attachment portions 111A and 111B, for fixing the detection electrodes, are securely welded to the outer wall of the center portion of the measurement pipe 12 perpendicular to the magnetic pole cores 14A and 14B.
First and second electrodes 112A and 112B are located next to the electrode attachment portions 111A and 111B, so that their electrodes are exposed through the lining member 19 and face the interior of the measurement pipe 12.
A first signal line 118A, extending from the first electrode 112A, is passed through the magnetic pole core 14B and is twisted together with a second signal line 118B on the second electrode 112B side. In order to twist the first signal line 118A and the second signal line 118B within the shortest distance possible, this structure is designed so that only the first signal line 118A is passed through the magnetic pole core 14B.
Disclosed in JP-A-2004-354279 is a technique whereby in order to prevent deformation of the lining, a groove is formed in the inner wall near the end face of a measurement pipe separate from the center of a measurement pipe, and a lining member is locked by the groove.
Disclosed in JP-A-2002-048612 is a technique whereby, in order to prevent deformation of the lining, a cylindrical locking plate wherein multiple holes are formed is arranged inside a lining member to lock the lining member.
Also, refer to JP-A-2004-294176.
However, the following problems are encountered with the first related small-diameter electromagnetic flowmeter shown in FIGS. 13 and 14.
Since the lining member 9 provided for the inner wall of the stainless steel measurement pipe 2 is formed of a fluoroplastic, the lining member 9 is not secured to the measurement pipe 2. However, since the lining member 9 is also deposited in the gaps 6A and 6B surrounding the magnetic pole cores 4A and 4B, movement of the measurement pipe 2 in the axial direction due to changes in the temperature is limited, but the measurement pipe 2 can still be moved in the radial direction. Therefore, using this structure, adequate locking effects are not obtained that can prevent deformations of the lining member 9 in the radial direction that are due to temperature fluctuations and pressure changes.
Therefore, for a measurement pipe having a small diameter of 15 mm or less, the ratio of deformations in the lining member 9 caused by temperature or pressure changes relative to the internal diameter is increased, and errors increase. Especially, deformation of the lining member 9 near the detection electrodes provides a great effect because the rate at which electromotive force is conducted to the detection electrodes is increased.
In order to prevent deformation of the lining member, in JP A-2004-354279, a structure is disclosed wherein a groove is formed in the inner wall of the measurement pipe near the end face to lock the lining member. However, with this structure, the lining near the detection electrodes that greatly influences measurement accuracy can not be secured.
Furthermore, for the second related small-diameter electromagnetic flowmeter shown in FIGS. 15 and 16, since the internal diameter of the spool pipe (the measurement pipe) is small, the insertion of a lining locking punch plate is difficult. Therefore, the internal lining diameter tends to change as the temperature of a fluid fluctuates, and the measurement accuracy is greatly affected by the change in the temperature of the fluid.
In addition, a rise in magnetic flux density is slowed due to an eddy current that is generated at a magnetic pole and affects the frequency property of the magnetic circuit. Further, since the first signal line penetrates a magnetic pole core, differential noise equivalent to a linkage dimension is generated only along the first signal line. When differential noise in the magnetic pole core is converged slowly and converging of the differential noise is still not satisfactory during signal sampling, differential noise is retained in the first signal. Further, since there is a difference in the amount of noise between the first and the second signals, the shifting distance at the zero point may be increased.
In JP-A-2002-048612, a structure is disclosed wherein the cylindrical locking plate, in which multiple holes are formed, is inserted inside the lining member. For this structure, however, there is no space in the small-diameter electromagnetic flowmeter for the insertion of a cylindrical locking plate, and this structure can not be provided for a small-diameter electromagnetic flowmeter.